Method for growing nitride semiconductor crystals, nitride semiconductor device, and method for fabricating the same

ABSTRACT

A method for growing nitride semiconductor crystals according to the present invention includes the steps of: a) forming a first metal single crystal layer on a substrate; b) forming a metal nitride single crystal layer by nitrifying the first metal single crystal layer; and c) epitaxially growing a first nitride semiconductor layer on the metal nitride single crystal layer.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method for growing nitridesemiconductor crystals, a nitride semiconductor device and a method forfabricating the same.

[0002] Nitride semiconductors such as GaN, InN and AlN are materialssuitably used for blue-light-emitting semiconductor laser devices andnumerous types of semiconductor devices, e.g., transistors operating ata high speed at an elevated temperature.

[0003] Various methods have been suggested to form a single crystallayer of a nitride semiconductor suitable for these semiconductordevices.

[0004] For example, according to a conventional technique, a nitridesemiconductor layer (e.g., an AlN layer) is directly deposited on asingle crystal substrate of sapphire (Al₂O₃) or Si by a metalorganicvapor phase epitaxy (abbreviated to “MOVPE” and also called a“metalorganic chemical vapor deposition (MOCVD)”) process. The nitridesemiconductor layer formed by this method, however, has poor surfacemorphology and is likely to crack, resulting in a lower yield. Thus,this method has not been put into practice. Cracking is probably causeddue to a thermal stress resulting from a difference in thermal expansioncoefficient between a single crystal substrate and a nitridesemiconductor layer during the process of lowering the depositiontemperature of the nitride semiconductor layer (about 1000° C. for AlN)to room temperature.

[0005] Another technique of forming a single crystalline nitridesemiconductor layer was developed later as disclosed in JapaneseLaid-Open Publications Nos. 4-297023 and 7-312350. According to thistechnique, an amorphous or polycrystalline nitride semiconductor layer(i.e., a GaN or Ga_(1−a)Al_(a)N (where 0<a≦1) layer) is once formed on asingle crystal substrate of sapphire or silicon at a relatively lowtemperature by an MOVPE process. Thereafter, the nitride semiconductorlayer is heated to form a partially single crystalline buffer layer andthen nitride semiconductor layers for a semiconductor device areepitaxially grown on the buffer layer.

[0006] A light-emitting device disclosed in Japanese Laid-OpenPublication No. 6-177423 is known as an exemplary semiconductor deviceusing a nitride semiconductor layer formed on a buffer layer. As shownin FIG. 14, this light-emitting device 900 includes: a buffer layer 95of polycrystalline or amorphous GaN or Ga_(1−a)Al_(a)N (where 0<a≦1); ann-type Ga_(1−b)Al_(b)N (where 0≦b<1) cladding layer 96; an n-typeIn_(x)Ga_(1−x)N (where 0<x<0.5) active layer 97; and a p-typeGa_(1−c)Al_(c)N (where 0≦c<1) cladding layer 98, which are stacked inthis order on a sapphire substrate 92.

[0007] The crystal growing technique for the buffer layer 95 is alsodisclosed in Japanese Laid-Open Publications Nos. 4-297023 and 7-312350identified above. Specifically, according to the method disclosed inthese references, GaN or Ga_(1−a)Al_(a)N (where 0<a≦1) crystals aregrown at a temperature ranging from 200° C. to 900° C., both inclusive,by an MOVPE process to form the buffer layer 95. In accordance with thismethod, part of the buffer layer 95 is turned into single crystalsduring a process of raising the temperature after the buffer layer 95 ofpolycrystalline Ga_(1−a)Al_(a)N (where 0<a≦1) has been deposited on thesapphire substrate 92 at a low temperature and before a nitridesemiconductor crystal layer, e.g., the n-type Ga_(1−b)Al_(b)N (where0≦b<1) cladding layer 96, is deposited at a temperature of about 1000°C.

[0008] The present inventors minutely analyzed the cross-section ofnitride semiconductor crystals, which had been grown on a sapphiresubstrate at a low temperature by the conventional technique, using atransmission electron microscope. As a result, we found that the nitridesemiconductor crystal layer, which had been formed by the prior artcrystal growing technique, had a lot of dislocations and that thelifetime of a semiconductor device including such a nitridesemiconductor layer was short.

[0009] In the conventional method for fabricating a semiconductordevice, it seems to be only a small region of the buffer layer 95 withina plane of the sapphire substrate 92 that is turned into single crystalsduring the temperature raising process before the nitride semiconductorcrystal layers are grown. Thus, it is considered that, in the remainingregion of the buffer layer 95 that is not turned into single crystals,the polycrystals have poorly aligned orientations to generate a largenumber of dislocations (or other defects) in the interface between thesapphire substrate 92 and the buffer layer 95. And such dislocationswould grow to reach the nitride semiconductor crystal layers (i.e., thecladding layer 96, active layer 97 and cladding layer 98 in this case).We found that the density of dislocations in the nitride semiconductorcrystal layers was as high as 10⁹ cm⁻², thus adversely shortening thelife of the semiconductor device.

[0010] Still another technique of forming an AlN buffer layer bynitrifying (in this specification, to “nitrify” means “to combine withnitrogen or its compounds”) the surface of a sapphire single crystalsubstrate was suggested in Japanese Laid-Open Publication No. 63-178516,for example. In accordance with this technique, however, the bufferlayer is also likely to crack or a lot of dislocations are also createdin the buffer layer as in the prior art method just described. Thus,this technique has not been put into practice, either.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is providing a method forgrowing nitride semiconductor crystals with the number of dislocationscreated in a nitride semiconductor crystal layer reduced, a highlyreliable semiconductor device with a longer lifetime, and a method forfabricating the same.

[0012] A method for growing nitride semiconductor crystals according tothe present invention includes the steps of: a) forming a first metalsingle crystal layer on a substrate; b) forming a metal nitride singlecrystal layer by nitrifying the first metal single crystal layer; and c)epitaxially growing a first nitride semiconductor layer on the metalnitride single crystal layer.

[0013] The present invention also provides a method for fabricating anitride semiconductor device including a semiconductor multilayerstructure and a pair of electrodes for applying a voltage to thesemiconductor multilayer structure. In this method, the step of formingthe semiconductor multilayer structure includes the step of epitaxiallygrowing the first nitride semiconductor layer by the method of thepresent invention for growing nitride semiconductor crystals.

[0014] A nitride semiconductor device according to the present inventionincludes: a single crystal substrate; a metal nitride single crystallayer formed by nitrifying a metal single crystal layer on the singlecrystal substrate; a semiconductor multilayer structure including afirst nitride semiconductor layer epitaxially grown on the metal nitridesingle crystal layer; and a pair of electrodes for applying a voltage tothe semiconductor multilayer structure.

[0015] Another nitride semiconductor device according to the presentinvention includes: a single crystal substrate with conductivity; ametal nitride single crystal layer formed by nitrifying a metal singlecrystal layer on the single crystal substrate; a semiconductormultilayer structure including a first nitride semiconductor layerepitaxially grown on the metal nitride single crystal layer; and a pairof electrodes formed to face each other on respective surfaces of thesingle crystal substrate and the semiconductor multilayer structure,which are interposed between the surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIGS. 1A, 1B and 1C are cross-sectional views illustrating amethod for growing nitride semiconductor crystals according to anexemplary embodiment of the present invention.

[0017]FIGS. 2A, 2B and 2C are cross-sectional views illustrating amethod for growing nitride semiconductor crystals according to anotherembodiment of the present invention.

[0018]FIGS. 3A, 3B and 3C are cross-sectional views illustrating amethod for growing nitride semiconductor crystals according to stillanother embodiment of the present invention.

[0019]FIGS. 4A, 4B and 4C are cross-sectional views illustrating amethod for growing nitride semiconductor crystals according to yetanother embodiment of the present invention.

[0020]FIGS. 5A, 5B, 5C and 5D are cross-sectional views illustrating amethod for growing nitride semiconductor crystals according to yetanother embodiment of the present invention.

[0021]FIGS. 6A, 6B, 6C and 6D are cross-sectional views illustrating amethod for growing nitride semiconductor crystals according to yetanother embodiment of the present invention.

[0022]FIG. 7 is a cross-sectional view schematically illustrating alight-emitting device 100 in a first specific example of a firstembodiment according to the present invention.

[0023]FIGS. 8A, 8B and 8C are cross-sectional views schematicallyillustrating a method for fabricating the light-emitting device 100shown in FIG. 7.

[0024]FIG. 9 is a graph illustrating respective relationships betweenoperating time and variation in operating current for the light-emittingdevices of the present invention and a conventional light-emittingdevice.

[0025]FIG. 10 is a cross-sectional view schematically illustrating alight-emitting device 200 in a first specific example of a secondembodiment according to the present invention.

[0026]FIGS. 11A, 11B and 11C are cross-sectional views schematicallyillustrating a method for fabricating the light-emitting device 200shown in FIG. 10.

[0027]FIG. 12 is a cross-sectional view schematically illustrating alight-emitting device 300 in a second specific example of the secondembodiment according to the present invention.

[0028]FIG. 13 is a cross-sectional view schematically illustrating alight-emitting device 400 in a third specific example of the secondembodiment according to the present invention.

[0029]FIG. 14 is a cross-sectional view schematically illustrating aconventional light-emitting device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] In a method for growing nitride semiconductor crystals accordingto the present invention, first, a metal single crystal layer is formedon a substrate, and then a metal nitride single crystal layer is formedby nitrifying the metal single crystal layer. Thereafter, nitridesemiconductor layers are epitaxially grown on the resulting metalnitride single crystal layer. In the nitrification process step, themetal single crystal layer need not be nitrified entirely.Alternatively, only part of the metal single crystal layer may benitrified and then nitride semiconductor layers may be epitaxially grownon a metal nitride single crystal layer formed around the surface of themetal single crystal layer. As a further alternative, another metalsingle crystal layer, different from the metal single crystal layer tobe nitrified, may be formed on the substrate and then the metal singlecrystal layer to be nitrified may be formed thereon.

[0031] The metal nitride single crystal layer, on which nitridesemiconductor layers are to be epitaxially grown, functions as aconventional buffer layer, thus improving the crystallinity of thenitride semiconductor layers. Such a buffer layer, made of a metalnitride formed by nitrifying the metal single crystals, is a singlecrystal layer with a much smaller number of defects than apolycrystalline layer or layer formed by turning part of apolycrystalline layer into single crystals in the prior art.Accordingly, nitride semiconductor layers with a low density ofdislocations may be deposited thereon by an epitaxy process.

[0032] In addition, compared to a conventional crystal-growingtechnique, the creation of cracks in the metal nitride single crystallayer or the nitride semiconductor layers formed thereon can besuppressed or virtually prevented. The creation of cracks can besuppressed probably by the following mechanism. Firstly, in accordancewith the thermal hysteresis during the process steps of forming themetal single crystal layer and nitrifying the metal single crystallayer, thermal stress, which is caused between the metal nitride singlecrystal layer and the substrate or the metal single crystal layer can bereduced. Secondly, since the interfacial state between the metal singlecrystal layer or the metal nitride single crystal layer and thesubstrate is different from that resulting from the conventionaltechnique, the stress can be relaxed, or the generation of the stresscan be suppressed.

[0033] The metal single crystal layer may be formed by a knowntechnique. For example, the metal single crystal layer may beepitaxially grown on a single crystal substrate by an ionized clusterbeam (ICB) process or a sputtering technique. Methods for growing ametal single crystal layer by an ICB process, ICB apparatuses and growthconditions are disclosed, for example, by H. Inokawa et al., Jpn. J.Appl. Phys. 24 (1985), pp. L173-L174, I. Yamada et al., J. Appl. Phys.56 (1986), pp. 2746-2750 and K. Yamada, edited by Japan Surface ScienceAssociation, “Thin-film Designing with Ion beams”, Section 5.5, pp.90-95, Kyoritsu Shuppan, 1991. A method for growing a metal singlecrystal layer by a sputtering technique is described, for example, by S.Yokoyama et al., Jpn. J. Appl. Phys. 32 (1993), pp. L283-L286. Accordingto the ICB process, in particular, a metal single crystal layer ofexcellent quality can be formed (the interfacial state between the metalsingle crystal layer and the single crystal substrate would also begood). In addition, in accordance with the ICB process, a metal singlecrystal layer can be epitaxially grown on a single crystal substratewith a relatively large lattice mismatch (e.g., about 25% or more). Thedocuments cited above are hereby incorporated by reference as thosedisclosing a method for epitaxially growing a metal single crystallayer, which is applicable to the embodiments of the present invention.

[0034] According to a method for epitaxially growing a metal singlecrystal layer on a single crystal substrate, various types of singlecrystal substrates may be used. The single crystal substrate may eitherbe a dielectric (insulator) or have electrical conductivity(semiconductor or conductor). If a conductive substrate is used, thenthe structure of the semiconductor device can be advantageouslysimplified. This point will be detailed in describing embodiments of amethod for fabricating a semiconductor device.

[0035] If a nitride semiconductor device is fabricated in accordancewith the method for growing nitride semiconductor crystals according tothe present invention, the creation of cracks in a nitride semiconductorlayer can be suppressed and the density of defects in the layer can bereduced. Accordingly, a highly reliable nitride semiconductor devicewith a long lifetime can be fabricated.

[0036] Hereinafter, a method for growing nitride semiconductor crystalsaccording to an exemplary embodiment of the present invention will bedescribed with reference to FIGS. 1A through 6. In all the drawingsreferred to in the following description, components with similar basicfunctions will be identified by the same reference numeral for the sakeof simplicity

[0037]FIGS. 1A, 1B and 1C are cross-sectional views illustratingrespective process steps for growing nitride semiconductor crystals inan exemplary embodiment of the present invention.

[0038] First, as shown in FIG. 1A, a metal single crystal layer 24 isformed on a substrate 22. For example, a single crystal substrate isused as the substrate 22 and the metal single crystal layer 24 isepitaxially grown on the single crystal substrate 22 by an ICB process,which may be carried out as disclosed in the documents cited above. Forinstance, the ICB process may be performed at room temperature within anambient at a pressure of about 1×10⁻⁹ Torr (i.e., about 1.4×10⁻⁷ Pa) orless. Before this epitaxial growth process step is performed, a processstep of cleaning the surface of the single crystal substrate 22 may becarried out.

[0039] The single crystal substrate 22 may be made of: insulator singlecrystals of sapphire, spinel, magnesium oxide, zinc oxide, chromiumoxide, lithium niobium oxide, lithium tantalum oxide or lithium galliumoxide; semiconductor single crystals represented bySi_(1−s−t)Ge_(s)C_(t) (where 0≦s, t≦1 and 0≦s+t≦1) or A_(1−u)B_(u)(where 0<u<1, A is one of Al, Ga and In and B is one of As, P and Sb);or metal single crystals of hafnium, for example. The metal singlecrystal layer 24 to be epitaxially grown on the single crystal substrate22 may be made of Al_(1−x−y)Ga_(x)In_(y) (where 0≦x, y≦1 and 0≦x+y<1).

[0040] Next, as shown in FIG. 1B, the metal single crystal layer 24 isnitrified, thereby forming the metal nitride single crystal layer 25.This nitrification process step may be performed by heating the metalsingle crystal layer 24 within an ambient of a compound containingnitrogen. The compound containing nitrogen is preferably hydrazine(N₂H₄) or ammonium (NH₃). Hydrazine is particularly preferable, becausehydrazine has higher nitrification ability than ammonium and can shortenthe nitrification time or lower the nitrification temperature.

[0041] The nitrification temperature can be appropriately set dependingon the necessity. However, the upper limit of the nitrificationtemperature is preferably lower than the melting point of the metalsingle crystal layer 24. This is because if the metal single crystallayer 24 is heated at a temperature equal to or higher than the meltingpoint thereof for a long time, then the metal single crystal layer 24melts and the crystal structure thereof collapses. In such a situation,the metal nitride layer, formed by the nitrification, is sometimes anon-single crystal layer or a crystal layer with a large number ofdislocations. Accordingly, in order to form a metal nitride singlecrystal layer of good quality, the metal single crystal layer ispreferably nitrified at a temperature lower than the melting point ofthe metal single crystal layer by about 100° C. or more. Thenitrification temperature has no particular lower limit. However, sincethe nitrification reaction of a metal is an Arrhenius-type reaction, thehigher the nitrification temperature, the shorter the time taken tonitrify the metal single crystal layer. For example, in nitrifying ametal single crystal layer of Al_(1−x−y)Ga_(x)In_(y), the nitrificationtemperature is preferably about 200° C. or more within the hydrazineambient or about 400° C. or more within the ammonium ambient. If thenitrification temperature is set at such a value, a metal single crystallayer with a thickness of several tens nanometers can be nitrifiedwithin several tens minutes. Since the metal nitride single crystallayer 25, which is formed by nitrifying the metal single crystal layer24, is thicker than the original metal single crystal layer 24, thethickness of the layer 25 is emphasized in FIG. 1B.

[0042] Then, a nitride semiconductor layer 26 is epitaxially grown onthe resulting metal nitride layer 25 by a known technique. For example,a layer made of a nitride represented as Al_(1−s−t)Ga_(s)In_(t)N (where0≦s, t≦1 and 0≦s+t≦1) may be epitaxially grown as the nitridesemiconductor layer 26. Naturally, the composition of the nitridesemiconductor layer 26 may be different from that of the metal nitridelayer 25. Since the metal nitride single crystal layer 25 has a smallnumber of dislocations, the nitride semiconductor layer 26, which isepitaxially grown thereon, is also a single crystal layer with a smallnumber of dislocations. In addition, compared to a conventionalcrystal-growing technique, the creation of cracks in the metal nitridesingle crystal layer 25 or the nitride semiconductor layer 26 formedthereon can be suppressed or virtually prevented. For example, accordingto the conventional crystal-growing technique, if a GaN layer isepitaxially grown on an AlN buffer layer deposited on an Si singlecrystal substrate at a high temperature (e.g., about 1000° C.), adistance between cracks, which are generated in the AlN buffer layer andthe GaN layer, is about 20 μm on average. On the other hand, if the GaNlayer is epitaxially grown on an AlN layer obtained by nitrifying an Almetal single crystal layer, a distance between cracks is about 2 mm to30 mm on average. An average distance between cracks, which aregenerated in the nitride semiconductor layer formed according to thecrystal-growing method of the present invention, is about 10 mm or more.Therefore, according to the crystal-growing method of the presentinvention, semiconductor devices can be fabricated with a good yield.

[0043] Another exemplary embodiment of a method for growing nitridesemiconductor crystals is illustrated in FIGS. 2A, 2B and 2C. Thisembodiment is different from the embodiment shown in FIGS. 1A, 1B and 1Cin the nitrification process step shown in FIG. 2B. Specifically, in thestep shown in FIG. 2B, the metal single crystal layer 24 is nitrifiedand metal atoms diffuse from the metal single crystal layer 24 into thesubstrate 22 to form a metal diffused layer 22 a within the surface ofthe substrate 22 (i.e., the interface between the metal nitride singlecrystal layer 25 and the substrate 22).

[0044] The probability of diffusion of the metal atoms is dependent onthe combination of materials for the single crystal substrate 22 and themetal single crystal layer 24. For example, if the single crystalsubstrate 22 is made of silicon or a semiconductor represented asA_(1−u)B_(u) (where 0<u<1, A is one of Al, Ga and In and B is one of As,P and Sb) and the metal single crystal layer 24 is made of Al or analloy containing Al, more specifically, Al_(1−x−y)Ga_(x)In_(y), then Alatoms are likely to diffuse into the substrate 22 to form the metaldiffused layer 22 a easily. For example, if Al is used to form the metalsingle crystal layer 24 and the resultant Al single crystal layer isnitrified at about 550°C. for about an hour, then a metal diffused layer22 a with a thickness of about 1 nm is obtained.

[0045] It is considered that this metal diffused layer 22 a improves theadhesion between the substrate 22 and the metal nitride single crystallayer 25 and relaxes a stress resulting from a difference in thermalexpansion coefficient therebetween. In addition, the metal diffusedlayer 22 a can also reduce the thermal contact resistance between thesubstrate 22 and the multilayer structure formed thereon. Furthermore,when the single crystal substrate 22 and the metal nitride singlecrystal layer 25 both have conductivity, the metal diffused layer 22 acan constitute an ohmic contact therebetween.

[0046] Still another exemplary embodiment of a method for growingnitride semiconductor crystals is illustrated in FIGS. 3A, 3B and 3C.This embodiment is different from the embodiment shown in FIGS. 1A, 1Band 1C in the nitrification process step shown in FIG. 3B. Specifically,in the step shown in FIG. 3B, only a part of the metal single crystallayer 24 is nitrified to form the metal nitride single crystal layer 25.The thickness of that part of the metal single crystal layer 24 to benitrified can be controlled by adjusting the nitrification time, forexample.

[0047] By partially leaving the metal single crystals 24 between thesingle crystal substrate 22 and the metal nitride single crystal layer25 without completely nitrifying the metal single crystal layer 24, thethermal contact resistance between the substrate 22 and the multilayerstructure formed thereon can be reduced. In addition, a stress createdbetween the single crystal substrate 22 and the metal nitride singlecrystal layer 25 can be relaxed by the metal single crystal layer 24.This is probably because the elastic modulus of a metal is generallylower than that of a nitride of the metal.

[0048] Yet another exemplary embodiment of a method for growing nitridesemiconductor crystals is illustrated in FIGS. 4A, 4B and 4C. Thisembodiment is different from the embodiment shown in FIGS. 1A, 1B and 1Cin the nitrification process step shown in FIG. 4B. Specifically, in thestep shown in FIG. 4B, only a part of the metal single crystal layer 24is nitrified to form the metal nitride single crystal layer 25, andmetal atoms diffuse from the metal single crystal layer 24 into thesubstrate 22 to form a metal diffused layer 22 a within the surface ofthe substrate 22 (i.e., the interface between the metal single crystallayer 24 and the substrate 22). As already described for the embodimentshown in FIGS. 2A, 2B and 2C, the metal diffused layer 22 a is formedsometimes easily but sometimes not, dependent on the combination ofmaterials for the single crystal substrate 22 and the metal singlecrystal layer 24. By using the above-exemplified combination ofmaterials and controlling the thickness of that part of the metal singlecrystal layer 24 to be nitrified, the structure shown in FIG. 4B can beobtained. As already described for the embodiment shown in FIGS. 3A, 3Band 3C, the thickness of that part of the metal single crystal layer 24to be nitrified can be controlled by adjusting the nitrification time,for example.

[0049] Yet another exemplary embodiment of a method for growing nitridesemiconductor crystals is illustrated in FIGS. 5A, 5B, 5C and 5D. Thisembodiment is different from the foregoing embodiments in that anadditional metal single crystal layer 23 is formed before the metalsingle crystal layer 24 to be nitrified is formed over the substrate 22as shown in FIG. 5A.

[0050] The metal single crystal layer 23, as well as the metal singlecrystal layer 24 of the foregoing embodiments, may be formed by a knowntechnique. For example, a single crystal substrate is prepared as thesubstrate 22 and the metal single crystal layer 23 is epitaxially grownthereon by an ICB process, for example. A metal material for the metalsingle crystal layer 23 is preferably Au or an alloy containing Au(e.g., an alloy of Au and Ge). By forming this additional metal singlecrystal layer 23, the thermal contact resistance between the substrate22 and the multilayer structure formed thereon can be reduced.

[0051] If the single crystal substrate 22 is made of a semiconductorrepresented as Si_(1−s−t)Ge_(s)C_(t) (where 0≦s, t≦1 and 0≦s+t≦1) orAl_(1−u)B_(u) (where 0<u<1, A is one of Al, Ga and In and B is one ofAs, P and Sb) where the metal single crystal layer 23 is made of Au oran alloy containing Au, then metal atoms constituting the metal singlecrystal layer 23 diffuse into the single crystal substrate 22 to form ametal diffused layer therein during the process step of nitrifying themetal single crystal layer 24. As a result, not only thermal contactresistance but also electrical contact resistance can be reduced betweenthe single crystal substrate 22 and the semiconductor multilayerstructure formed thereon. In this case, part of the atoms constitutingthe metal single crystal layer 23 may be diffused. Alternatively, asshown in FIGS. 6A, 6B, 6C and 6D, a metal diffused layer 22 a may beformed by diffusing all the atoms constituting the metal single crystallayer 23 into the single crystal substrate 22 during the step of formingthe metal nitride single crystal layer 25 through the nitrification ofthe metal single crystal layer 24. According to this method, the metalsingle crystal layer 23 disappears (see FIG. 6C). In order to eliminatethe metal single crystal layer 23 through diffusion, the thickness ofthe metal single crystal layer 23 is preferably about 3 nm or less.Also, the temperature and time for the process step of nitrifying themetal single crystal layer 24 may be set based on the degree ofdiffusion of the metal single crystal layer 23. For example, if atoms inthe metal single crystal layer 23 should be continuously diffused afterthe nitrification reaction of the metal single crystal layer 24 is over,heating may be continued.

[0052] This embodiment may be combined with any of the foregoingembodiments. For example, if the metal single crystal layer 23 is madeof Au or an alloy containing Au and the single crystal substrate 22 ismade of a semiconductor represented as Si_(1−s−t)Ge_(s)C_(t) orA_(1−u)B_(u), then metal atoms diffuse from the metal single crystallayer 23 into the single crystal substrate 22 to form the metal diffusedlayer 22 a as in the embodiment shown in FIG. 2B. In this case, if themetal single crystal layer 23 is sufficiently thin (e.g., about 3 nm orless), then all the metal atoms constituting the metal single crystallayer 23 diffuse into the single crystal substrate 22 and substantiallyno metal single crystal layer 23 is left. In this structure, the AlNsingle crystal layer 25 with satisfactorily aligned crystal orientationsis formed on the sapphire substrate 22. Accordingly, the density ofdefects or dislocations can be reduced both in the interface between thesapphire substrate 22 and the AlN single crystal layer 25 and in then-type Ga_(0.9)Al_(0.1)N cladding layer 26, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 and p-type GaN contact layer 29,which are stacked thereon. In this structure, increase in resistance dueto a Schottky barrier generated by a semiconductor/metal interface canbe prevented, since there is no semiconductor/metal interface.

[0053] Also, after the metal single crystal layers 23 and 24 have beenformed, only a part of the metal single crystal layer 24 may benitrified as described for the embodiment shown in FIG. 3B. Furthermore,after the metal single crystal layers 23 and 24 have been formed, themetal diffused layer 22 a may be formed and only a part of the metalsingle crystal layer 24 may be nitrified to leave the metal singlecrystal layer 24 between the metal nitride single crystal layer 25 andthe metal single crystal layer 23 as described for the embodiment shownin FIG. 4B. In any combination, the effects of the embodiment shown inFIG. 2B, 3B or 4B can be additionally attained.

[0054] Since a nitride semiconductor layer having (0001) principalsurface is generally used in a semiconductor device, it is preferable toform a metal nitride layer having (0001) principal surface so that anitride semiconductor layer having (0001) principal surface can beepitaxially grown on the metal nitride layer. More specifically, when asingle crystal substrate, made of semiconductor single crystalsrepresented by Si_(1−s−t)Ge_(s)C_(t) (where 0≦s, t≦1 and 0≦s+t≦1) orA_(1−u)B_(u) (where 0<u<1, A is one of Al, Ga and In and B is one of As,P and Sb), is used, an Al_(1−x−y)Ga_(x)In_(y)N (where 0≦x, y≦1 and0≦x+y<1) single crystal layer having (0001) principal surface can beobtained by nitrifying an Al_(1−x−y)Ga_(x)In_(y) layer having (111)principal surface epitaxially grown on (111) plane of the single crystalsubstrate made of Si_(1−s−t)Ge_(s)C_(t) or A_(1−u)B_(u).

[0055] Hereinafter, specific embodiments of fabricating a semiconductordevice in accordance with the foregoing method for growing nitridesemiconductor crystals will be described. In the following illustrativeembodiments, a light-emitting device (i.e., a semiconductor laser diode)is fabricated as an exemplary semiconductor device. However, the presentinvention is not limited to those specific embodiments, but isapplicable to various other semiconductor devices like a field effecttransistor (FET).

EMBODIMENT 1

[0056] In a first exemplary embodiment of the present invention, alight-emitting device 100 is formed using a substrate with noconductivity.

Specific Example 1-1

[0057] As shown in FIG. 7, the light-emitting device 100 includes: anAlN single crystal layer 25 (thickness: 10 nm); an n-typeGa_(0.9)Al_(0.1)N cladding layer 26 (thickness: 1 μm); a multiplequantum well (MQW) active layer 27; a p-type Ga_(0.9)Al_(0.1)N claddinglayer 28 (thickness: 0.5 μm); and a p-type GaN contact layer 29(thickness: 0.1 μm), which are stacked in this order on a sapphiresubstrate 22. The MQW active layer 27 is formed by alternately stackingten pairs of undoped In_(0.2)Ga_(0.8)N layers (thickness: 5 nm) andundoped GaN layers (thickness: 5 nm). Of these layers, the lowermostundoped GaN layer is in contact with the n-type Ga_(0.9)Al_(0.1)Ncladding layer 26.

[0058] This semiconductor multilayer structure, including the claddinglayer 26, active layer 27, cladding layer 28 and contact layer 29, whichare formed on the AlN single crystal layer 25 has been subjected to amesa-etching process. Through this etching process, a pair of electrodes32 a and 32 b for applying a voltage to the semiconductor multilayerstructure are formed on the contact layer 29 and on the cladding layer26, respectively.

[0059] In this structure, the AlN single crystal layer 25 withsatisfactorily aligned crystal orientations is formed on the sapphiresubstrate 22. Accordingly, the density of defects or dislocations can bereduced both in the interface between the sapphire substrate 22 and theAlN single crystal layer 25 and in the n-type Ga_(0.9)Al_(0.1)N claddinglayer 26, MQW active layer 27, p-type Ga_(0.9)Al_(0.1)N cladding layer28 and p-type GaN contact layer 29, which are stacked thereon.

[0060] The light-emitting device 100 may be fabricated in accordancewith the crystal-growing method shown in FIGS. 1A through 1C. A methodfor fabricating this light-emitting device 100 will be described withreference to FIGS. 8A, 8B and 8C.

[0061] First, as shown in FIG. 8A, an Al single crystal layer 24 isdeposited on a sapphire substrate 22 by an ICB process. Next, as shownin FIG. 8B, the Al single crystal layer 24 is nitrified to be an AlNsingle crystal layer 25. The nitrification may be performed by reactingnitrogen components, which are included in a nitrogen compound such ashydrazine or ammonium contained in an appropriate carrier gas (e.g., H₂gas), with the Al single crystal layer 24 while the temperature of thesapphire substrate 22 is kept at 550° C., which is about 100° C. lowerthan the melting point of Al single crystals (i.e., 660° C.).

[0062] Thereafter, as shown in FIG. 8C, an n-type Ga_(0.9)Al_(0.1)Ncladding layer 26 doped with Si, an MQW active layer 27, a p-typeGa_(0.9)Al_(0.1)N cladding layer 28 doped with Mg and a p-type GaNcontact layer 29 doped with Mg are stacked in this order on the AlNsingle crystal layer 25 by an MOVPE process. In this process step,crystals for the n-type Ga_(0.9)Al_(0.1)N cladding layer 26, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 and p-type GaN contact layer 29 aregrown at 1000° C., while crystals for the MQW active layer 27 are grownat 800° C.

[0063] The resulting semiconductor multilayer structure, including therespective layers 26, 27, 28 and 29, is partially etched, therebyexposing the n-type Ga_(0.9)Al_(0.1)N cladding layer 26. Finally,respective ohmic electrodes 32 a and 32 b are formed on the p-type GaNcontact layer 29 and on the n-type Ga_(0.9)Al_(0.1)N cladding layer 26to complete the light-emitting device 100. The electrode 32 a may beformed of, for example, Ni/Au, and the electrode 32 b may be formed of,for example, Ti/Au by an electron beam deposition method.

[0064] In this structure, since the AlN single crystal layer 25 isformed by nitrifying the Al single crystal layer 24, the AlN singlecrystal layer 25 can be formed over the entire surface of the sapphiresubstrate 22. Accordingly, the crystallinity of the n-typeGa_(0.9)Al_(0.1)N cladding layer 26, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 and p-type GaN contact layer 29,which are stacked on the AlN single crystal layer 25, can be improved.

[0065] The cross section of the light-emitting device 100 according tothe first specific example of the first embodiment was observed with atransmission electron microscope (TEM). As a result, the density ofdefects or dislocations in the n-type Ga_(0.9)Al_(0.1)N cladding layer26, MQW active layer 27, p-type Ga_(0.9)Al_(0.1)N cladding layer 28 andGaN contact layer 29 was 1.0×10⁵/cm², which is about {fraction(1/10,000)}compared to a conventional light-emitting device.

Specific Example 1-2

[0066] A light-emitting device according to a second specific example ofthe first embodiment includes an Al_(0.9)Ga_(0.1)N single crystal layer25 (thickness: 5 nm), the n-type Ga_(0.9)Al_(0.1)N cladding layer 26,MQW active layer 27, p-type Ga_(0.9)Al_(0.1)N cladding layer 28 andp-type GaN contact layer 29, which are stacked in this order on anMgAl₂O₄ (spinel) substrate 22 as shown in FIG. 7.

[0067] In this structure, the Al_(0.9)Ga_(0.1)N single crystal layer 25with satisfactorily aligned crystal orientations is formed on the spinelsubstrate 22. Accordingly, the density of defects or dislocations can bereduced in both the interface between the spinel substrate 22 and theAl_(0.9)Ga_(0.1)N single crystal layer 25, and the n-typeGa_(0.9)Al_(0.1)N cladding layer 26, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 and p-type GaN contact layer 29,which are stacked thereon.

[0068] A method for fabricating this light-emitting device 100 will bedescribed with reference to FIGS. 8A, 8B and 8C again.

[0069] First, as shown in FIG. 8A, an Al_(0.9)Ga_(0.1) alloy singlecrystal layer 24 is deposited to be 5 nm thick on a spinel substrate 22by an ICB process. Next, as shown in FIG. 8B, the Al_(0.9)Ga_(0.1) alloysingle crystal layer 24 is nitrified to be an Al_(0.9)Ga_(0.1)N singlecrystal layer 25. The nitrification may be performed by reactingnitrogen components, which are included in a nitrogen compound such ashydrazine or ammonium contained in an appropriate carrier gas (e.g., H₂gas), with the Al_(0.9)Ga_(0.1) alloy single crystal layer 24 while thetemperature of the spinel substrate 22 is kept at 500° C.

[0070] Thereafter, as shown in FIG. 8C, n-type Ga_(0.9)Al_(0.1)Ncladding layer 26 doped with Si, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 doped with Mg and p-type GaN contactlayer 29 doped with Mg are stacked in this order on theAl_(0.9)Ga_(0.1)N single crystal layer 25 as in the first specificexample. The resulting semiconductor multilayer structure, including therespective layers 26, 27, 28 and 29, is partially etched, therebyexposing the n-type Ga_(0.9)Al_(0.1)N cladding layer 26. Finally,respective ohmic electrodes 32 a and 32 b are formed on the p-type GaNcontact layer 29 and on the n-type Ga_(0.9)Al_(0.1)N cladding layer 26.

[0071] In this structure, since the Al_(0.9)Ga_(0.1)N single crystallayer 25 is formed by nitrifying the Al_(0.9)Ga_(0.1) alloy singlecrystal layer 24, the Al_(0.9)Ga_(0.1)N single crystal layer 25 can beformed over the entire surface of the spinel substrate 22. Accordingly,the crystallinity of the n-type Ga_(0.9)Al_(0.1)N cladding layer 26, MQWactive layer 27, p-type Ga_(0.9)Al_(0.1)N cladding layer 28 and p-typeGaN contact layer 29, which are stacked on the Al_(0.9)Ga_(0.1)N singlecrystal layer 25, can be improved.

[0072] The cross section of the light-emitting device 100 according tothe second specific example of the first embodiment was observed with aTEM. As a result, the density of defects or dislocations in the n-typeGa_(0.9)Al_(0.1)N cladding layer 26, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 and GaN contact layer 29 was1.0×10⁵/cm², which is about {fraction (1/10,000)} compared to aconventional light-emitting device.

[0073]FIG. 9 illustrates respective lifetimes of the light-emittingdevices 100 according to the first and second specific examples of thefirst embodiment (hereinafter, identified by E1 and E2, respectively)and a conventional light-emitting device C, which are all operated at atemperature of 70° C. with an optical output of 5 mW. In FIG. 9, thecurves E1, E2 and C indicate respective relationships between theoperating time and a variation in operating current per unit time of thelight-emitting devices E1, E2 and C. In FIG. 9, as the variation ΔI/Δt,which is a variation of the operating current with the operating time,comes closer to 1, a light-emitting device deteriorates to a lesserdegree and can operate for a longer time. As shown in FIG. 9, in thelight-emitting devices E1 and E2 of the present invention, ΔI/Δt isstill close to 1 even after these devices have been operated for 10,000hours. In contrast, after the conventional light-emitting device C hasbeen operated for 5,000 hours, ΔI/Δt greatly deviates from 1.Accordingly, the light-emitting devices E1 and E2 of the presentinvention have much longer lifetimes, and are a lot more reliable, thanthe conventional light-emitting device C. It should be noted that theoscillation wavelengths of these light-emitting devices were all 420 nm.

[0074] In the foregoing specific examples, the same effects can beattained if the sapphire or spinel substrate 22 is replaced with asingle crystal substrate of MgO, ZnO, Cr₂O₃, LiNbO₃, LiTaO₃ or LiGaO₂.

[0075] As described above, the first embodiment of the present inventionprovides a semiconductor device with reduced defects or dislocations inthe interface between an insulating single crystal substrate and anitride semiconductor crystal layer, a longer lifetime and higherreliability and a method for fabricating the same.

[0076] In the foregoing illustrative embodiment, a method forfabricating a light-emitting device in accordance with the nitridesemiconductor crystal-growing method shown in FIGS. 1A through 1C hasbeen described. Alternatively, any of the other crystal-growing methodsshown in FIGS. 2A through 6D is also applicable. According to any ofthese methods, the same effects as those of the first embodiment can beattained.

EMBODIMENT 2

[0077] In a second exemplary embodiment of the present invention, alight-emitting device 200 is formed using a substrate with conductivity,which includes a semiconductor substrate and a conductor substrate madeof a metal, for example.

Specific Example 2-1

[0078] As shown in FIG. 10, the light-emitting device 200 according to afirst specific example of the second embodiment includes: an Al singlecrystal layer 24 (thickness: 8 nm); an AlN single crystal layer 25(thickness: 2 nm); an n-type Ga_(0.9)Al_(0.1)N cladding layer 26(thickness: 1 μm); an MQW active layer 27; a p-type Ga_(0.9)Al_(0.1)Ncladding layer 28 (thickness: 0.5 μm); and a p-type GaN contact layer 29(thickness: 0.1 μm), which are stacked in this order on an n-type Sisubstrate 22. The MQW active layer 27 is formed by alternately stackingtwenty pairs of undoped In_(0.2)Ga_(0.8)N layers (thickness: 5 nm) andundoped GaN layers (thickness: 5 nm). Of these layers, the lowermostundoped GaN layer is in contact with the n-type Ga_(0.9)Al_(0.1)Ncladding layer 26.

[0079] A pair of electrodes 32 a and 32 b for applying a voltage to thesemiconductor multilayer structure, including the n-type cladding layer26, active layer 27, p-type cladding layer 28 and contact layer 29,which are formed on the AlN single crystal layer 25, are formed on thecontact layer 29 and on the Si single crystal substrate 22,respectively, so as to face each other.

[0080] In this structure, the Al single crystal layer 24 withsatisfactorily aligned crystal orientations is formed on the n-type Sisingle crystal substrate 22, and the AlN single crystal layer 25 isformed thereon. Accordingly, the density of defects or dislocations canbe reduced in both the interface between the n-type Si single crystalsubstrate 22 and the Al single crystal layer 24, and the n-typeGa_(0.9)Al_(0.1)N cladding layer 26, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 and p-type GaN contact layer 29,which are stacked thereon. Also, heat generated in the MQW active layer27 can be directly dissipated through the n-type Si single crystalsubstrate 22. Furthermore, since an electrode can be formed on the backof the n-type Si single crystal substrate 22, an increased number oflight-emitting devices can be formed per substrate 22 compared to aconventional structure. That is to say, a light-emitting device can befabricated at a lower cost.

[0081] The light-emitting device 200 may be fabricated in accordancewith the crystal-growing method shown in FIGS. 3A through 3C. A methodfor fabricating this light-emitting device 200 will be described withreference to FIGS. 11A, 11B and 11C.

[0082] First, as shown in FIG. 11A, an Al single crystal layer 24 isdeposited to be 10 nm thick on an n-type Si single crystal substrate 22by an ICB process. Next, as shown in FIG. 8B, part of the Al singlecrystal layer 24 is nitrified to the depth of 2 nm as measured from thesurface thereof, thereby turning that part into an AlN single crystallayer 25 with the thickness of 2 nm. The nitrification may be performedby reacting nitrogen components, which are included in a nitrogencompound such as hydrazine or ammonium contained in an appropriatecarrier gas (e.g., H₂ gas), with the Al single crystal layer 24 whilethe temperature of the n-type Si single crystal substrate 22 is kept at550° C., which is about 100° C. lower than the melting point of Alsingle crystals (i.e., 660° C.).

[0083] Thereafter, as shown in FIG. 11C, n-type Ga_(0.9)Al_(0.1)Ncladding layer 26 doped with Si, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 doped with Mg and p-type GaN contactlayer 29 doped with Mg are stacked in this order on the AlN singlecrystal layer 25 by an MOVPE process. In this process step, crystals forthe n-type Ga_(0.9)Al_(0.1)N cladding layer 26, p-type Ga_(0.9)Al_(0.1)Ncladding layer 28 and p-type GaN contact layer 29 are grown at 1000° C.,while crystals for the MQW active layer 27 are grown at 800° C.

[0084] Finally, respective ohmic electrodes 32 a and 32 b are formed toface each other on the p-type GaN contact layer 29 and on the n-type Sisingle crystal substrate 22, thereby completing the light-emittingdevice 200. The ohmic electrode 32 b may be formed of, for example, Al,Ti or Pt, with an optional annealing step at about 300° C. to about 400°C. The ohmic electrode 32 b may be formed as described in Embodiment 1of the present invention.

[0085] In this structure, since the AlN single crystal layer 25 isformed by nitrifying the Al single crystal layer 24, the Al and AlNsingle crystal layers 24 and 25 can be formed over the entire surface ofthe n-type Si single crystal substrate 22. Accordingly, thecrystallinity of the n-type Ga_(0.9)Al_(0.1)N cladding layer 26, MQWactive layer 27, p-type Ga_(0.9)Al_(0.1)N cladding layer 28 and p-typeGaN contact layer 29, which are stacked on the AlN single crystal layer25, can be improved.

[0086] The cross section of the light-emitting device 200 according tothe first specific example of the second embodiment was observed with aTEM. As a result, the density of defects or dislocations in the n-typeGa_(0.9)Al_(0.1)N cladding layer 26, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 and p-type GaN contact layer 29 was1.0×10⁵/cm², which is about {fraction (1/10,000)} compared to aconventional light-emitting device.

Specific Example 2-2

[0087] A light-emitting device 300 according to a second specificexample of the second embodiment includes Al_(0.9)Ga_(0.1)N singlecrystal layer 25 (thickness: 5 nm), n-type Ga_(0.9)Al_(0.1)N claddinglayer 26, MQW active layer 27, p-type Ga_(0.9)Al_(0.1)N cladding layer28 and p-type GaN contact layer 29, which are stacked in this order onan n-type GaAs substrate 22 as shown in FIG. 12.

[0088] In this structure, the Al_(0.9)Ga_(0.1)N single crystal layer 25with satisfactorily aligned crystal orientations is formed on the n-typeGaAs substrate 22. Accordingly, the density of defects or dislocationscan be reduced in both the interface between the n-type GaAs substrate22 and the Al_(0.9)Ga_(0.1)N single crystal layer 25 and the n-typeGa_(0.9)Al_(0.1)N cladding layer 26, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 and p-type GaN contact layer 29,which are stacked thereon. Also, heat generated in the MQW active layer27 can be directly dissipated through the n-type GaAs substrate 22.Furthermore, since an electrode can be formed on the back of the n-typeGaAs substrate 22, an increased number of light-emitting devices can beformed per substrate 22 compared to a conventional structure. That is tosay, a light-emitting device can be fabricated at a lower cost.

[0089] The light-emitting device 300 is fabricated in accordance withthe crystal-growing method shown in FIGS. 1A through 1C. A method forfabricating this light-emitting device 300 will be described withreference to FIGS. 8A, 8B and 8C again.

[0090] First, as shown in FIG. 8A, an Al_(0.9)Ga_(0.1) alloy singlecrystal layer 24 is deposited to be 5 nm thick on an n-type GaAssubstrate 22 by an ICB process. Next, as shown in FIG. 8B, theAl_(0.9)Ga_(0.1) alloy single crystal layer 24 is nitrified to be anAl_(0.9)Ga_(0.1)N single crystal layer 25. The nitrification may beperformed by reacting nitrogen components, which are included in anitrogen compound such as hydrazine or ammonium contained in anappropriate carrier gas (e.g., H₂ gas), with the Al_(0.9)Ga_(0.1) alloysingle crystal layer 24 while the temperature of the n-type GaAssubstrate 22 is kept at 500° C. Thereafter, as shown in FIG. 8C, n-typeGa_(0.9)Al_(0.1)N cladding layer 26 doped with Si, MQW active layer 27,p-type Ga_(0.9)Al_(0.1)N cladding layer 28 doped with Mg and p-type GaNcontact layer 29 doped with Mg are stacked in this order on theAl_(0.9)Ga_(0.1)N single crystal layer 25 as in the first specificexample.

[0091] Finally, respective ohmic electrodes 32 a and 32 b are formed toeach face other on the p-type GaN contact layer 29 and the n-type GaAssubstrate 22, thereby completing the light-emitting device 300.

[0092] In this structure, since the Al_(0.9)Ga_(0.1)N single crystallayer 25 is formed by nitrifying the Al_(0.9)Ga_(0.1) alloy singlecrystal layer 24, the Al_(0.9)Ga_(0.1)N single crystal layer 25 can beformed over the entire surface of the n-type GaAs substrate 22.Accordingly, the crystallinity of the n-type Ga_(0.9)Al_(0.1)N claddinglayer 26, MQW active layer 27, p-type Ga_(0.9)Al_(0.1)N cladding layer28 and p-type GaN contact layer 29, which are stacked on theAl_(0.9)Ga_(0.1)N single crystal layer 25, can be improved.

[0093] The cross section of the light-emitting device 300 according tothe second specific example of the second embodiment was observed with aTEM. As a result, the density of defects or dislocations in the n-typeGa_(0.9)Al_(0.1)N cladding layer 26, MQW active layer 27, p-typeGa_(0.9)Al_(0.1)N cladding layer 28 and GaN contact layer 29 was1.0×10⁵/cm², which is about {fraction (1/10,000)}compared to aconventional light-emitting device.

[0094] Respective lifetimes of the light-emitting devices 200 and 300according to the first and second specific examples of the secondembodiment, which were both operated at a temperature of 70° C. with anoptical output of 5 mW, are substantially the same as those of thelight-emitting devices E1 and E2 shown in FIG. 9. That is to say, in thelight-emitting devices 200 and 300 of the second embodiment, ΔI/Δt isstill close to 1 even after these devices have been operated for 10,000hours. In contrast, after the conventional light-emitting device C hasbeen operated for 5,000 hours, ΔI/Δt greatly deviates from 1.Accordingly, the light-emitting devices 200 and 300 of the secondembodiment also have much longer lifetimes, and are a lot more reliable,than the conventional light-emitting device C. It should be noted thatthe oscillation wavelengths of these light-emitting devices were all 420nm.

[0095] In the foregoing specific examples of the second embodiment, thesame effects are attained if the n-type Si single crystal substrate orn-type GaAs substrate 22 is replaced with a semiconductor single crystalsubstrate with conductivity such as n-type GaAs substrate or n-type SiCsubstrate. Also, a p-type semiconductor single crystal substrate withconductivity or a conductor single crystal substrate made of a metalsuch as hafnium may be used instead. Among various metals, hafniumsingle crystals are preferable, because the lattice constant of hafniumsingle crystals is close to that of nitride semiconductor singlecrystals.

[0096] As described above, according to the second embodiment of thepresent invention, a metal single crystal layer and a nitridesemiconductor single crystal layer are formed in this order on aconductive single crystal substrate and then semiconductor layers areformed thereon. Accordingly, heat radiation can be improved and thedensity of defects or dislocations in the semiconductor layers can bereduced. Furthermore, since an electrode can be formed on the back ofthe conductive single crystal substrate, semiconductor devices can befabricated at a lower cost.

Specific Example 2-3

[0097] A light-emitting device 400 according to a third specific exampleof the second embodiment includes AlN single crystal layer 25(thickness: 5 nm), n-type Ga_(0.9)Al_(0.1)N cladding layer 26, MQWactive layer 27, p-type Ga_(0.9)Al_(0.1)N cladding layer 28 and p-typeGaN contact layer 29, which are stacked in this order on an n-type GaAssubstrate 22 as shown in FIG. 13. In addition, a metal diffused layer 22a is formed within the n-type GaAs substrate 22 in the vicinity of thesurface thereof closer to the AlN single crystal layer 25. The metaldiffused layer 22 a is formed by diffusing an alloy containing Au.

[0098] A pair of electrodes 32 a and 32 b for applying a voltage to thesemiconductor multilayer structure, including the n-type cladding layer26, active layer 27, p-type cladding layer 28 and contact layer 29,which are formed on the AlN single crystal layer 25, are formed on thecontact layer 29 and on the n-type GaAs substrate 22, respectively, soas to face each other.

[0099] The light-emitting device 400 is fabricated in accordance withthe crystal-growing method shown in FIGS. 6A through 6D.

[0100] First, an n-type GaAs single crystal substrate 22, of which theprincipal surface is (111) plane, is prepared. On the (111) plane of thesingle crystal substrate 22, a metal single crystal layer 23 of an Au/Gealloy is epitaxially grown to be about 1 nm thick by an ICB process asshown in FIG. 6A. The principal surface of the resulting AuGe singlecrystal layer 23 is also (111) plane.

[0101] Next, an Al single crystal layer 24 doped with Si at about 10¹⁸cm⁻³ is epitaxially grown to be about 20 nm thick on the (111) plane ofthe AuGe single crystal layer 23 by an ICB process. The principalsurface of the resulting Al single crystal layer 24 is also (111) plane.

[0102] In epitaxially growing the metal single crystal layers 23 and 24by an ICB process, an ICB apparatus, which includes source gas supplies(i.e., AuGe and Si-doped Al source gas supplies) for supplying therespective sources for the metal single crystal layers 23 and 24 withina single chamber and can control the flow rates of these source gasesfrom the source gas supplies using a shutter, for example, is preferablyused. If such an ICB apparatus is used, a high-purity film can bedeposited, since there is no need to take out the specimens from thechamber (i.e., without breaking the vacuum within the chamber or causingleakage). The epitaxy of the metal single crystal layers 23 and 24 bythe ICB process may be performed at room temperature, for example.

[0103] Then, while the Al single crystal layer 24 is nitrified, AuGeatoms are diffused from the AuGe single crystal layer 23 into the n-typeGaAs substrate 22. In this process step, the GaAs single crystalsubstrate 22 is heated up to a temperature lower than the respectivemelting points of the GaAs single crystal substrate 22 itself and the Alsingle crystal layer 24 (e.g., 550° C.) and a nitrogen-containingcompound gas is supplied into the chamber. The nitrogen-containingcompound is preferably hydrazine or ammonium. In particular, sincehydrazine has high nitrification ability, hydrazine is preferable inview of the productivity. This is because the nitrification time can beshortened and the nitrification temperature can be lowered in such acase. By nitrifying the Al single crystal layer 24 with a thickness of20 nm for about 10 to about 15 minutes, the AlN single crystal layer 25is formed. The principal surface of the resulting AlN single crystallayer 25 is (0001) plane. Since the thickness increases as a result ofthe nitrification, the AlN layer 25 is illustrated in FIG. 6B as beingthicker than the Al layer 24. Also, in this process step, AuGe atomsdiffuse into the n-type GaAs substrate 22 to form an AuGe diffused layer22 a as shown in FIG. 6C. To diffuse the AuGe atoms, the heatingtemperature and time may be adequately adjusted during the nitrificationprocess step. Even after the nitrification reaction is substantiallyover, heating may be continued for the diffusion purpose only.

[0104] Thereafter, a cladding layer 26 of n-type Ga_(0.9)Al_(0.1)Nsingle crystals is epitaxially grown on the AlN single crystal layer 25by an ICB process as shown in FIG. 6D. Subsequently, a doubleheterojunction semiconductor multilayer structure, including the n-typecladding layer 26, active layer 27, p-type cladding layer 28 and contactlayer 29, is formed thereon by epitaxy as in the foregoing embodiments.And then electrodes 32 a and 32 b are formed to face each other on thecontact layer 29 and on the n-type GaAs substrate 22, respectively. As aresult, the light-emitting device 400 shown in FIG. 13 is completed. Thecrystal structure of the semiconductor multilayer structure ishexagonal, and a laser device can be formed with crystals cleaved on thefacets of the cavity.

[0105] In the AuGe diffused layer 22 a, which is formed within then-type GaAs substrate 22 in the vicinity of the surface thereof closerto the n-type AlN layer 25, Ge atoms function as donors. Accordingly,the Ge atoms decrease the electrical resistance in the interface betweenthe n-type GaAs substrate 22 and the n-type AlN layer 25. Thus, thislight-emitting device 400, in which the pair of electrodes 32 a and 32 bare provided with the conductive substrate 22 and the semiconductormultilayer structure interposed therebetween, can have its operatingvoltage reduced.

[0106] In this third specific example of the second embodiment, if ap-type GaAs substrate is used as the conductive single crystal substrate22 and AuNi single crystal layer and Mg-doped (e.g., about 0.5 mol %) Alsingle crystal layer are used as respective metal single crystal layers23 and 24, then a p-type AlN layer 25 can be formed over the p-type GaAssubstrate 22. Accordingly, a light-emitting device can be fabricatedwith a reversed arrangement of conductivity types in the doubleheterostructure. In this reversed arrangement, an AuNi diffused layer 22a is formed within the p-type GaAs substrate 22 in the vicinity of thesurface thereof closer to the p-type AlN layer 25. Ni atoms in the AuNidiffused layer 22 a function as acceptors, thus decreasing theelectrical resistance in the interface between the p-type GaAs substrate22 and the p-type AlN layer 25. As a result, the light-emitting devicecan operate at a lower voltage.

[0107] In this specific example, the metal single crystal layer 23 maybe made of element Au or any other alloy containing Au.

[0108] According to the inventive method for growing nitridesemiconductor crystals, a nitride semiconductor layer is epitaxiallygrown on a metal nitride single crystal layer obtained by nitrifying ametal single crystal layer. Therefore, a nitride semiconductor layer isobtained with a much smaller number of dislocations or defects than thatformed by a conventional crystal growing method.

[0109] In addition, a highly reliable nitride semiconductor device witha longer lifetime can be obtained by fabricating the semiconductordevice using the method for growing nitride semiconductor crystalsaccording to the present invention. The inventive method for growingnitride semiconductor crystals is advantageously applicable to a methodfor fabricating a blue light emitting laser diode.

[0110] While the present invention has been described in a preferredembodiment, it will be apparent to those skilled in the art that thedisclosed invention may be modified in numerous ways and may assume manyembodiments other than that specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention which fall within the true spirit andscope of the invention.

In the claims:
 1. A nitride semiconductor device comprising: a singlecrystal substrate; a metal nitride single crystal layer formed bynitrifying a metal single crystal layer on the single crystal substrate;a semiconductor multilayer structure including a first nitridesemiconductor layer epitaxially grown on the metal nitride singlecrystal layer; and a pair of electrodes for applying a voltage to thesemiconductor multilayer structure.
 2. A nitride semiconductor devicecomprising: a single crystal substrate with conductivity; a metalnitride single crystal layer formed by nitrifying a metal single crystallayer on the single crystal substrate; a semiconductor multilayerstructure including a first nitride semiconductor layer epitaxiallygrown on the metal nitride single crystal layer; and a pair ofelectrodes formed to face each other on respective surfaces of thesingle crystal substrate and the semiconductor multilayer structure,which are interposed between the surfaces.
 3. The device of claim 1 ,further comprising a first metal single crystal layer on the singlecrystal substrate, and wherein the metal nitride crystal layer is formedby nitrifying a second metal single crystal layer epitaxially grown onthe first metal single crystal layer.
 4. The device of claim 1 , whereinthe single crystal substrate comprises a metal diffused layer in whichmetal atoms have diffused from the metal nitride single crystal layer.5. The device of claim 1 , wherein the single crystal substratecomprises a metal diffused layer in which metal atoms have diffused fromthe first metal single crystal layer.
 6. The device of claim 1 , whereinthe single crystal substrate is made of Si_(1−s−t)Ge_(s)C_(t)(where 0≦s,t≦1 and 0≦s+t≦1).
 7. The device of claim 1 , wherein the single crystalsubstrate is made of A_(1−u)B_(u), where 0<u<1, A is one of Al, Ga andIn and B is one of As, P and Sb.
 8. The device if claim 1 , wherein thesingle crystal substrate is made of a material selected from the groupconsisting of: sapphire; spinel; magnesium oxide; zinc oxide; chromiumoxide; lithium niobium oxide; lithium tantalum oxide; and lithiumgallium oxide.
 9. The device of claim 5 , wherein the first metal singlecrystal layer is made of Au or an alloy containing Au.
 10. The device ofclaim 1 , wherein the metal nitride single crystal layer is made ofAl_(1−x−y)Ga_(x)In_(y)N (where 0≦x, y≦1 and 0≦x+y<1).
 11. The device ofclaim 10 , wherein the single crystal substrate is a single crystalsubstrate of silicon, of which the principal surface is (III) plane, andwherein the metal nitride single layer is formed on the (III) plane andthe principal surface of the metal nitride single crystal layer is(0001) plane.